280 SCIENCE AND PRACTICE OF DAIRYING. 



effect. This is, however, no discredit to the makers, who do not know, and 

 could not know, in constructing their apparatus, the cause of the defect. 

 If it be desired in working with the apparatus in use to make sure that 

 all the most dangerous pathogenic bacteria, and the vegetative forms of 

 nearly all the remaining kinds of bacteria, have been destroyed, care must 

 be taken, in the first place, that the milk be subjected in the apparatus to 

 75 C., and further, that the hot milk should be kept in special vessels for 

 30 minutes at a temperature of over 70 C. There is no Pasteurizing 

 apparatus, therefore, as yet, which gives in a convenient, simple, and 

 certain manner properly sterilized milk. Whether it is possible to manu- 

 facture such an apparatus, without doing away with the continuous flow 

 of milk, must be decided by practical makers. Possibly, as H. Bitters has 

 pointed out, the Pasteurizing apparatus of the future will be constructed 

 in such a manner that the milk will not be heated in a continuous flow, 

 but that it will be heated intermittently, and for a definite and high tem- 

 perature, for a certain time. 



131. Sterilized Unthickened Milk. The perfect sterilization of 

 milk, that is, the destruction of all spores in it, is extremely difficult. 

 It can be effected, if desired, in a twofold manner. In the first place, 

 it may be effected by heating the milk in strong closed vessels for 

 several hours at a temperature of 110 C., or for 30 minutes at a 

 temperature of 130 C.; or, secondly, by heating the milk on eight 

 consecutive days, for two hours each day, at a temperature of 

 65 C., and keeping the milk in the interim period at a tempera- 

 ture of 40 C., that is, by intermittent sterilization, a method first 

 employed by Tyndall. In the first method of treatment the value 

 of the milk is lessened, since the particular qualities which are 

 specially prized in fresh milk are entirely lost. The second method 

 of treatment is so inconvenient, and consumes so much time, that 

 although by this method the value of the milk is little affected, it 

 cannot be carried out on a large scale. The perfect sterilization of 

 milk by either method is of little practical importance, and can only 

 be carried out in the laboratory for experimental purposes. In 

 practice, one must be content with the empirical method of steri- 

 lization, in order to change the milk as little as possible, and be 

 satisfied with destroying the vegetative kinds of bacteria, along 

 with such pathogenic bacteria as may be present, and with acting 

 upon the lasting spores, which may not be destroyed, in such a way 

 that their capacity for development may be weakened, and that 

 they may at ordinary animal heat only exercise a dangerous action 



